The invention relates to a power source comprising an arrangement for restricting a short-circuit current. A power source according to the invention includes a primary-side switch block, transformer unit and a rectifier which comprises at least one switching transistor. The invention further relates to a method for restricting the short-circuit current in a power source comprising a primary-side switch block, transformer unit and a rectifier which comprises at least one switching transistor.
There are known power sources which include an arrangement for restricting a short-circuit current. In one arrangement, current is switched off by a transistor on the basis of a rise in the voltage across a sensing resistor.
From the prior art we further know of galvanically isolated power sources producing a d.c. voltage from a battery. Such power sources are utilized at least in connection with fixed telecommunication network equipment. A simple Buck switcher as well as flyback and forward converters are known. The most common of these is perhaps the forward converter. It uses switching transistors to periodically switch off the current or reverse its polarity at the primary winding of an isolation transformer, and a rectifier to rectify the current generated at the secondary winding. Rectification is realized by a passive diode construction or by synchronous switching transistors.
In closer examination, a synchronous forward converter includes a switcher for switching the d.c. voltage of the battery into a variable-polarity voltage for the primary winding of the isolation transformer, and a rectifier for switching, synchronized with the switcher, the a.c. voltage obtained from the secondary winding of the isolation transformer back into a single-polarity voltage which is rectified by an inductance-capacitance connection. The switcher in the power source mainly consists of a pulse-width modulator and primary-side transistors controlled by the former. Similarly, the synchronous rectifier comprises secondary-side transistors controlled by a switch circuit which in turn is controlled by the pulse-width modulator. The control pulses of the pulse-width modulator are adjusted on the basis of the output signal of the whole switcher circuit, which output signal is fed back to the pulse-width modulator. The adjustment of pulses affects the switching state of the primary-side transistors in the isolation transformer. Restriction of possible short-circuit current is realized by shortening the switching pulses of the transistors used for connecting the d.c. voltage to the primary winding.
A problem with diode rectifiers of galvanically isolated power sources according to the prior art is the considerable amount of power required by the diodes, which could be, for example, about 9% of the transmitted power in cases where the output voltage is less than 3.3 volts.
Another problem with galvanically isolated power sources according to the prior art and with their synchronous rectifiers is the large short-circuit current in the output conductors. The largeness of the current is mainly due to the lower pulse width limit of the pulse-width modulator controlling the transistors which provide input to the primary winding of the isolation transformer. This lower limit causes a minimum coupling to the isolation transformer, and the current thus generated at the secondary winding is further conducted to the power source output in half cycles. When the output of the rectifier in the power source is shorted the current increases significantly at the secondary side and may result in damages to components and even to the circuit board. If the damaged sub-entity is an essential part of the device, the unit must be replaced. Momentary accidental short circuits may occur when installing devices.
An object of the invention is to provide a power source and method for restricting a short-circuit current, which power source comprises a primary-side switch block, transformer unit and a rectifier which includes at least one switching transistor.
A power source according to the invention with an arrangement for restricting the short-circuit current and comprising a primary-side switch block, transformer unit and a rectifier which includes at least one switching transistor is characterized in that
the control for the at least one switching transistor in the rectifier is arranged so as to be cut off in an overload situation, and
the rectifier comprises at least one switch element connected in parallel with the switching transistor, whereby rectification at the secondary side is arranged so as to be realized in an overload situation by means of the said switch element.
The method according to the invention for restricting the short-circuit current of a power source comprising a primary-side switch block, transformer unit and a rectifier including at least one switching transistor is characterized in that
the control for the said at least one switching transistor is cut off in an overload situation, and
rectification at the secondary side is realized in an overload situation substantially by means of at least one switch element connected in parallel with the secondary-side switching transistor.
Advantageous embodiments according to the invention are specified in the dependent claims.
In the arrangement according to the invention for restricting the short-circuit current in a synchronous rectifier there are, connected in parallel with the switching transistors of the secondary winding, diodes and a circuit that cuts off the control to the switching transistors in a short-circuit situation. The diodes can provide rectification in a short-circuit situation when the secondary-side switching transistors are non-conductive. Advantageously the output of the circuit includes an inductance whereby the pulse-width modulator and, further, the primary-side switching transistors can be controlled according to the output voltage.
The secondary-side switching transistor control cut-off is based on a variable-pulse-height current supply from the primary-side switching transistor control line through an amplifier, capacitor, isolation transformer and rectifier bridge. When the primary-side switching transistor control pulses remain low, the switch circuit controlling the secondary-side switching transistors will not get sufficient operating voltage but is turned off.
Return to normal operation is, however, automatic as the diodes according to the invention maintain, by means of inductance, control to the primary-side switching transistors, and when the short-circuit is removed the control returns to normal and the switch circuit of the secondary-side switching transistors begins functioning again. In normal operation, the diodes according to the invention are momentarily conductive as the status of the signal produced by the pulse-width modulator changes, whereafter, as the switching transistor goes into conduction, the current through the diodes is nearly zero when all the current flows through the transistor.